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Green Sturgeon: The Bay Delta's Disappearing Prehistoric Sea Monster

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Adult green sturgeon | Photo: René Reyes, U.S. Bureau of Reclamation
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Want to get an evolutionary biologist to roll her eyes at you? Here's one way: refer to a wildlife species as a "living fossil." It's a bit of popular jargon that obscures the fact that every living thing, person or palm tree or protozoan, has gone through the same 3.8 billion years of evolution.

Still, some species seem so similar to their ancestors of millions of years ago that it's hard not to reach for the term. The green sturgeon, one of the Bay Delta's largest fish, is emphatically among them. A member of the group Chondrostei, green sturgeons resemble in some striking ways the ancestors of modern bony fish. They have rigid bonelike plates instead of scales, they mainly have cartilage where other fish would have bone, and they have Alien-like protrusible jaws. Chondrostei have been around since before there were dinosaurs, and its not hard to imagine fish very much like the green sturgeon plying Mesozoic waters.

But don't let that fool you. Green sturgeons and their ancestors have been evolving this whole time, and the fish is finely adapted to conditions the Bay Delta had just a few decades ago. But now those conditions are changing too rapidly, and the sturgeon is having trouble keeping up.

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The green sturgeon's size helps bolster the prehistoric sea monster image. Easily capable of reaching lengths between 4.5 and 6.5 feet at maturity, weighing up to 350 pounds, and living for 70 years, the green sturgeon -- Acipenser medirostris -- spawns in bays, estuaries, and coastal rivers throughout the Northern Pacific from China and Russia eastward to Canada and the United States.

The population of green sturgeon that lives in the Bay Delta is part of what federal fisheries biologists have termed the Southern distinct population segment (DPS) of the species, made up of all green sturgeon populations south of the Eel River. Though it had historically been found in a few other estuaries and coastal rivers, the Sacramento River and Bay Delta now holds the sole remaining spawning population of green sturgeon in the Southern DPS.

And the fish is by no means secure even there. As was the case with the Bay Delta's Chinook salmon, the green sturgeon saw most of its best habitat made inaccessible during the 20th century by dams.

Like the salmon, the green sturgeon is anadromous: it spawns in freshwater, generally as far upstream as it can reach given appropriate water conditions, then spends much of its adult life in salt water. As adults, green sturgeon from the Bay Delta often venture as far north as the Alaska coast, and occasionally south to Mexico.

Compared to other sturgeon, including the significantly larger white sturgeon of the Bay Delta (which can reach 20 feet in length and weigh half a ton), green sturgeon spend a lot of time at sea. Most other sturgeon stick either to freshwater exclusively, or, like the white sturgeon, stay in either brackish or protected saline waters. (Bay Delta white sturgeon usually spend most of their lives in San Pablo Bay.)

The green sturgeon, by contrast, spends three or so years in freshwater after hatching out. The larvae grow rapidly, provisioned with an extra large yolk sac. By the time they're ready to venture out into the Pacific, they range from one to three feet long.

Not much is known about green sturgeon's habits, including diet, while they're in the open ocean. In the Bay Delta, they use their extensibly-jawed mouths to suction up zooplankton and small animals, including freshwater and saltwater shrimp, native clams, copepods, and isopods, but -- according to one study -- the fish seem to forego the invasive Asian clam, Corbicula fluminea.

Corbicula, as mentioned in this article on the invasive clams taking over the Bay Delta, is part of the reason for a massive collapse in the Bay Delta's ecological productivity, due to the clam's voracious consumption of phytoplankton -- the base of the Bay Delta aquatic food chain. If the sturgeon found Corbicula palatable, that could bring it more food in the long term as well as in the short term. Then again, as both Corbicula and its saltwater colleague Corbula amurensis accumulate toxic selenium in their tissues, perhaps the green sturgeon is better off not trying them. The clams and their selenium load have been implicated in reproductive injury to the related white sturgeon.

The collapse of the Bay Delta's pelagic ecosystem is just one of the problems green sturgeon face. Cutting off most of the fish's historic spawning habitat was a huge blow: the Bay Delta's green sturgeon now mainly spawn in the lower Sacramento River, though some spawning has been observed in the Feather River as well.

Sturgeon are also hard-hit by water pollution, including runoff from farms. Bycatch by commercial fishing boats has been an issue as well. So has deliberate catching of the sturgeon, by poachers intent on selling black market caviar.

Before settlement of California by Europeans, green sturgeon were abundant enough to supply a commonly used source of food for Native people in the Delta, who caught the monstrous fish with handwoven nets and spears. A century and a half of trying to live with us settlers has brought the green sturgeon's numbers so low that the National Marine Fisheries Service listed the Southern DPS of the green sturgeon as Threatened under the U.S. Endangered Species Act in 2006. Fishing for the Southern DPS was halted in California a year later.

Now, though information is sparse, some suggest that fewer than 50 adult green sturgeon may remain in the southern DPS.

Which means we might be on the verge of seeing 3.8 billion years of evolution come to an end for the Bay Delta's green sturgeon.

For ongoing environmental coverage in March 2017 and afterward, please visit our show Earth Focus, or browse Redefine for historic material.
KCET's award-winning environment news project Redefine ran from July 2012 through February 2017.

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